Well logging, also known as borehole logging, is the practice of making a detailed record (a well log) of the geologic formations penetrated by a borehole. Resistivity logging is a method of well logging that works by characterizing the rock or sediment in a borehole by measuring its electrical resistivity. Resistivity is a fundamental material property which represents how strongly a material opposes the flow of electric current. Most rock materials are essentially insulators, while their enclosed fluids are conductors. Hydrocarbon fluids are an exception, because they are almost infinitely resistive. When a formation is porous and contains salty water, the overall resistivity will be low. When the formation contains hydrocarbons, or contains very low porosity, its resistivity will be high. High resistivity values may indicate a hydrocarbon bearing formation.
In one aspect, embodiments disclosed herein relate a modular resistivity sensor disposable in a drillstring having a drill bit on a distal end for drilling a wellbore in a formation, the sensor including a sensor body having a longitudinal axis radially offset from a longitudinal axis of the drillstring, one or more transmitting coil antennas disposed along a length of the sensor body, one or more receiving coil antennas disposed along a length of the sensor body, and a calibrating device configured to calibrate signals generated from the receiving coil antennas.
In other aspects, embodiments disclosed herein relate to a method of making resistivity measurements of a formation from a wellbore being drilled by a tool having a drill bit disposed on a distal end, the method including providing a modular resistivity sensor comprising at least one transmitting coil antenna, one or more receiving coil antennas, and a calibrating coil antenna located longitudinally between the receiving coil antennas, firing the at least one transmitting coil antenna for generating a current loop in the formation, thereby inducing a first set of independent voltage signals in the receiving coil antennas related to a parameter of the formation, firing the calibrating antenna at a desired frequency, thereby inducing a second set of independent voltage signals in the receiving coil antennas, and calibrating the first set of independent voltage signals relative to the second set of voltage signals.
In yet other aspects, embodiments disclosed herein relate to a method of making resistivity measurements of a formation from a wellbore being drilled by a tool having a drill bit disposed on a distal end, the method including providing a modular resistivity sensor comprising at least one transmitting coil antenna, one or more receiving coil antennas, and a calibrating signal generator, firing the at least one transmitting coil antenna for generating a current loop in the formation, thereby inducing independent voltage signals in the receiving coil antennas related to a parameter of the formation, generating a voltage signal at a desired frequency in the calibrating signal generator, transmitting the generated voltage signal to electronics coupled to the receiving antennas, and calibrating the independent voltage signals induced in the receiving coil antennas.